This invention relates to cleaning filtration systems and, more specifically, to a mechanical apparatus for use in a filtration system having horizontally disposed filter elements to remove expended filter cake from the filter elements or septum.
Various types of filtration systems exist for clarifying liquids by removing solid particles (whether initially dissolved or suspended therein) and chemical contaminants, as well as to recover reclaimable materials from the liquid. These systems employ mechanical screening devices utilized as a support for a chemically adsorptive/absorptive material such that the solid and the chemical contaminants are adsorbed/absorbed by the surface of this material while the liquid passes through. As the contaminants are removed from the liquid, however, they accumulate on the surface of the filter as sludge, inhibiting the flow of the liquid therethrough, and eventually clogging the filter and necessitating the removal of the filter material or cake from the supporting screen or filter septum.
In such filtration systems, therefore, various filter cleaning means, such as brushes, scrapers and/or sprayers are used to mechanically remove the filter material or filter cake and sludge from the filter septum or screen. Other methods employed to remove spent filter cake and sludge from the filter elements, filter leaves, or circular filter tubes are vibration and backwashing, i.e., reversal of the liquid flow through the filter elements. However, backwashing is detrimental to the filter and reduces its life considerably, since most filters are constructed of fine mesh screen septums supported on the low pressure side which will distend and easily distort when subjected to pressure from that side. Vibration will also harm the support screen over a period of time.
In many applications, where the flow rate of liquid through the filter is slow, the filter cake has a tendency to fall from the septum where vertically disposed filters are used. Therefore, such applications require the use of horizontally disposed filter elements which cannot be easily cleaned by vibration or backwashing. Rotating such horizontal filters as high rates of speed to remove spent filter cake and sludge by centrifugal force often results in the septum being torn from the support material along with the expended filter cake which will require shutting down the operation to replace the filter element. Where the filter cake is strongly bonded to the septum, the entire cake may not be removed by centrifugal force, thereby resulting in the formation of uneven cake upon subsequent precoating of the filter septum.
Where brushes and scrapers have been used on horizontally disposed filters it has been found that as the filter is rotated against these radially extending brushes, filter cake and sludge accumulate or pile up, not only creating frictional forces which resist further movement, but also downward forces against the filter screen which tend to damage it permanently or force filter cake through the septum contaminating the clarified liquid. The resulting cleaning is unsatisfactory.
The above-mentioned methods are not efficient and do not maintain the outer surface of a filter element completely clean for long periods of time, which may result in the septum becoming slimed, blinded, or clogged. If the filter elements are only partially cleaned, the efficiency of the filtration process and the period in which the filter is in service and on line is substantially reduced.